Cheers all. Cake was nice, but is all gone now.
Fair play to the lady that makes them. She's done a few different cakes for us over the years, but this one did put a big grin on my face when my two gave it to me.

Pull the reservoir off from the cylinder. There is either a threaded lock under there or a jiggle pin that you have to pull out while pushing the piston back in a bit to line up the spool.
I'm sure one of the other members had this issue a few months back.

Finally, there is much rejoicing. Don't know whether to kick myself or attempt a cartwheel.
Stripped the caliper back off again earlier and everything was running free with it when off. Refitted it and there was no free float again and binding. So, off it came again, then I noticed a scuff mark on the underside when I was about to wind the piston back.
Fitted the caliper back on again, noting the scuff point and immediately it lost its float again. Lying under the car from behind I could then see that the casting was pressing against the axle bracket when bolted up and was stopping the caliper from moving freely.
If you look at the pics you can see the area that was scuffing, so I took the caliper back off again and filed off the high spot on the casting. Put it back on again and bingo, freeplay.
I haven't checked the other side, as it got late, but I'll definitely be giving it a look over to see if there's any rubbing.
Fingers crossed, that's it all sorted for my binding and transmission loss to back wheels on recent RR session.

Think you've hit the nail on the head there, Katana. Got the shop right too. It needs to come off again, definitely.
I'm not even stepping foot in the garage to look at it tonight. Think I'll just sit on my hole and watch tv with a cup of tea and some chocolate hobnobs.

Another 2 steps forward and then another 1 back today. Still not sorted as I've now come in to go visit my parents.
Stripped drivers seat out so I could lie upside down to get at the bias pedal box. Remote adjuster when being turned wasn't creating any movement on the bias bar. Then noticed that it was adjusted all the way over to 1 side with no thread at all on 1 end. Did a YouTube search and have now set it at 50/50 with equal thread showing on both. Remote adjuster now works, but I might remove that permanently, so nobody turns it. Then I'll put a small locknut on the bias rod and get it on a brake roller to check it when I come of Sorn after the winter.
Right, back to the binding:
Drivers side is running free and locking solid when pedal is pressed. This caliper is floating freely when pushed and pulled.
Passenger side is still binding with no pedal pressed and fully locking when applied. This caliper has no free float on it when pushed or pulled. Handbrake cable is disconnected and arm for that is in the full rest position.
I'm now confident that the pedal is not binding as rod is free at master cylinder and also because drivers wheel spins OK. Looks like it must be at the caliper, even though it's fairly new and done very little mileage.
Car is currently left at this state now in garage.

I can't confirm if the listing is right or wrong, as I've not purchased any drums.
There are other brands in there too and when you click on the drawing, it generally gives you the dimensions etc and also the OEM equivalent part number.
I suppose you would need an old drum to measure and confirm the correct fit though.

Wide awake as motion sensor has woken us up earlier. Nothing suspicious though, but after checking cameras I can't drop back off now.
On a plus side, these have popped up on the Book earlier and this lad has lots of bits for sale, with a lot more listed underneath. Way to much to screenshot: